Support MMS

chatty#30 (closed) is for MMS support in chatty. https://gitlab.freedesktop.org/mobile-broadband/ModemManager/-/issues/257 is for ModemManager. Before chatty#30 (closed) can be solved (or as part of solving that), we will need MMS support here in the connector. Also, it seems to me that most of the work will be here, not in ModemManager, just as mmsd handles MMS on ofono based phones.

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